A Diminished Role for the Sarcoplasmic Reticulum in Newborn Myocardial Contraction: Effects of Ryanodine

Abstract

ABSTRACT. Ryanodine, known to reduce Ca2+ release from the sarcoplasmic reticulum (SR), was used in conjunction with the single sucrose gap voltage clamp technique to study excitation-contraction coupling in papillary muscles isolated from newborn and adult rabbits. Ryanodine altered the components of voltage clamp-induced tension in the adult producing a pattern that closely resembled the normal newborn. The phasic component of tension in mature myocardium was markedly and significantly reduced by ryanodine. In addition, ryanodine significantly altered the voltage dependence of this component of tension, suggesting a change in its mechanism of generation. The appearance of a prominent phasic component of tension, absent normally, but produced in the newborn by CaZ' loading, was also abolished by ryanodine. These results support previous proposals that the phasic component of voltage clamp-induced tension in mammalian myocardium is produced by the release of Ca2+ from the SR. In the newborn, ryanodine caused a significantly smaller decrease in the ratio of phasic to tonic tension than in the adult, suggesting a much less significant role for the SR in excitation-contraction coupling in the younger age group. Although ryanodine had a significant effect on phasic tension, no alteration in the amplitude or voltage dependence of tonic tension, generated by transmembrane Ca2+ influx, could be demonstrated in either newborn or adult heart. This investigation suggests that in newborn myocardium, Ca2+ release from the SR plays a negligible role in excitation-contraction coupling, which depends, rather, on the influx of Ca2+ from the extracellular space. ( characterized by an early peak of tension (phasic tension) which then relaxes to a steady state level (1, 2). It has been postulated that the lack of phasic tension in the immature myocardium reflects structural and functional immaturity of the t-tubules and SR ( 1, 3). To further investigate this hypothesis, experiments were conducted using the alkaloid ryanodine in an attempt to selectively eliminate Ca2+ release from the SR. Ryanodine has been shown to diminish release of Ca2+ from the SR, either by directly inhibiting Ca2+ release mechanisms (4-6), or by enhancing Ca2+ emux, resulting in Ca2+ depletion of the SR (7, 8), with little or no effect on the amplitude of membrane Ca2+ currents (9, 10) or the function of contractile proteins (1 I). It has been demonstrated previously that the negative inotropic effect of ryanodine is less pronounced in immature than mature mammalian myocardium (1 2-14) suggesting that it may be a useful tool in studying the changes which occur in the mechanisms of myocardial excitation-contraction coupling with development. MATERIALS AND METHODSPreparation. Right ventricular papillary muscles, 0.3-0.7 mm in diameter, were dissected from seven newborn (0-10 days) and six adult New Zealand White rabbits. Muscles were isolated and placed in a three compartment single sucrose gap, voltage clamp chamber. The tendinous end of the papillary muscl...

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